Why was Mendeleev's law changed?

Generally

Humans have always tried to find an explanation for the complexity of the matter around us. We first thought that the elements of all matter are simply water, earth, fire and air. However, after some time and thanks to the development of physics and chemical experimentation technology, we have realized that matter is more complicated than it first appeared. 19th century chemists found it necessary to classify the newly discovered elements. The first way, also the most natural, was to classify them according to their atomic mass, but this classification did not reflect the differences and similarities of the individual elements. Many other classifications were adopted before they arrived and were used on the Periodic Table of the Elements.


Chronology of the different classifications of the elements

Döbereiner

This chemist established a relationship between the atomic mass of certain elements and their properties in 1817. He emphasized the existence of the similarities between groups of elements in groups of three, which are also known under the name "Triads". The chlorine, bromine, and iodine triad is an example. He pointed out that atomic mass is one of the three elements in the triad, meanwhile, between the other two. In 1850 we had 20 different traids to achieve the first coherent classification.

Chancourtois & Newlands

In 1862, Chancourtois, a French geologist, brought out a certain periodicity between elements. In 1864, Chancourtois and Newlands, an English chemist, published the Law of Octaves: the properties repeated after every eighth element. But this law cannot be applied to elements after calcium. This classification is not sufficient, but the periodic table of the elements can be redeveloped.

Meyer

In 1869 Meyer, a German chemist, brought out a certain periodicity in atomic volumes. Similar elements have the same atomic volume compared to other elements. Alkynic elements, for example, have a significant atomic volume.

Mendeleïev

In 1869, Mendeleïev, a Russian chemist, published a first version of his periodic table of the elements. This periodic table was the first coherent presentation on the similarity between the elements. He noticed that in the classification of the elements according to their atomic mass, a periodicity could be seen in similar properties. The first periodic table contained 63 elements.

This periodic table was designed to show the element periodicity. In this way the elements were classified vertically. The resulting horizontal groups represent the elements of the same family.

In order to confirm the law he believed in, he had to leave some voids behind.

Mendeleïev interpreted his periodic table in such a way that the periodicity of the elements emerges clearly. In this periodic table, the elements are classified vertically (in the current classification, they are arranged horizontally). The horizontal arrangement occurs regularly where certain elements and physical properties are repeated. In the vertical arrangement we find that the elements in their chemical properties mostly have the same chemical properties and similarities.

In order to follow the periodic law, in which Mendeleïev firmly believes, he should sometimes change the order according to the atomic mass distribution and leave certain places "cavities" behind.

He was sure that we would discover the last missing elements (the elements corresponding to the question marks in front of the relative atomic mass 45, 68, 70 and 180) which would confirm how well founded his theory was. More than that, he predicted the properties of the three missing elements by observing the properties of the four neighboring elements. These three elements (gallium, scandium and germanium) were discovered between 1875 and 1886. Every one of them had its properties predicted by a Russian chemist. By then, a great many scientists had accepted Mendeleïev's idea. But as soon as these predicted elements were discovered, they exhibited properties very similar to those predicted. Scientists recognized this usefulness from its periodic table.